A choke coil is used as an electronic equipment, which is employed in a controlling power supply for an office automation equipment, a solar electricity generation system, vehicles, and uninterruptible power supply units. As a core for such choke coil, a ferrite core or a dust core is used. The ferrite core has a disadvantage that the saturation magnetic flux density is small, while the dust core, which is manufactured by molding a metal powder, has a higher saturation magnetic flux density than that of the soft magnetic ferrite, and thus is excellent in DC superposition characteristics.
For meeting the requirements of improving energy conversion efficiency and achieving low heat generation, the dust core is needed to have magnetic properties in which a large magnetic flux density can be obtained by applying a small magnetic field, and further the energy loss can be made low in the variation of magnetic flux density. As a form of energy loss, there is a core loss (iron loss) that occurs when the dust core is used in an alternating magnetic field. The core loss (Pc) is expressed by the sum of a hysteresis loss (Ph) and an eddy current loss (Pe), as shown in the following Equation (1). The hysteresis loss is proportional to the operation frequency, and the eddy current loss (Pe) is proportional to the square of the operation frequency, as shown in the following Equation (2). Therefore, the hysteresis loss (Ph) is dominant in a low-frequency range, while the eddy current loss (Pe) is dominant in a high-frequency range. It is necessary to make the dust core having magnetic properties reducing the occurrence of the core loss (Pc).Pc=Ph+Pe  (1)Ph=Kh×f Pe=Ke×f2  (2)
wherein Kh is a hysteresis loss factor, Ke is an eddy current loss factor, and f is a frequency.
In order to reduce the hysteresis loss (Ph) of the dust core, a displacement of a magnetic domain wall should be facilitated by reducing the coercive force of the soft magnetic powder particle. Incidentally, the reduction of the coercive force also achieves the improvement of the initial permeability as well as the reduction of the hysteresis loss. As shown in the following Equation (3), the eddy current loss is inversely proportional to the resistivity of the core.Ke=k1Bm2t2/ρ  (3)
wherein k1 is a factor, Bm is a magnetic flux density, t is a particle size (or thickness of the plate material), and ρ is a resistivity.
From the above reason, pure iron, having small coercive force, has been widely used as soft magnetic powder particle. For example, it is known a method to use the pure iron as soft magnetic powder and making the impurity mass ratio to the soft magnetic powder 120 ppm or less, thereby reducing the hysteresis loss (e.g. see Patent document 1). Also, it is known a method to use the pure iron as soft magnetic powder and make an amount of manganese contained in the soft magnetic powder 0.013 wt % or less, thereby reducing the hysteresis loss (e.g. see Patent document 2). Besides, it is known a method in which the soft magnetic powder is heated before forming an insulation film thereon.
Furthermore, another method is known in which the hysteresis loss is reduced by heating the soft magnetic powder before forming an insulation film thereon. By this method, the stress existed in the soft magnetic particles can be eliminated, the defects in the crystal grain boundary can be eliminated, the crystal particles in the soft magnetic powder particles can be grown (enlarged), therefore a displacement of a magnetic domain wall should be facilitated and thus the coercive force of the soft magnetic powder particle can be reduced. For example, it is known a method in which heating process is performed in an inert atmosphere at 800° C. or more to a soft magnetic powder composed mainly from iron, containing 2-5 wt % Si, having average particle size of 30-70 μm, and having an average aspect ratio of 1-3. By this method, the crystal particles in the powder particles can be enlarged and the coercive force can be reduced, and thus the hysteresis loss can be reduced (see Patent document 3). Also, it is known a method in which the metal particles are mixed with spacer particles and the metal particles are separated from each other, thereby preventing the metal particles from sintering and bonding to each other (e.g. see Patent document 4).    Patent document 1: Japanese Patent Application Laid-open No. 2005-15914    Patent document 2: Japanese Patent Application Laid-open No. 2007-59656    Patent document 3: Japanese Patent Application Laid-open No. 2004-288983    Patent document 4: Japanese Patent Application Laid-open No. 2005-336513